Project Summary Cell size control is critical to all domains of life and changes in cell size are an important indicator of perturbations that impact the balance between cell growth and cell cycle progression. Using cell size as a lens, our work seeks to develop a clear, integrated, systems-level picture of the homeostatic regulatory circuits coordinating nutrient availability with cell growth and cell cycle progression in bacteria. In single celled organisms, these circuits ensure the efficient partitioning of limited resources, maximize proliferative potential, and preserve viability in response to changing environmental conditions. Defects in these circuits can be catastrophic, interfering with vital stress response mechanisms, severely impairing growth and dramatically reducing viability. Accruing data support a central role for a global regulator of biosynthetic capacity, the small molecule guanosine tetraphosphate (ppGpp), in the regulatory circuits coordinating nutrient availability with divergent aspects of bacterial physiology. Current projects focus on three aspects of these essential homeostatic regulatory networks in which ppGpp plays a role: 1) the mechanisms governing the balance between lateral cell wall growth and septal wall synthesis, 2) the signal transduction pathways coordinating cytoplasmic aspects of anabolic metabolism with lipid synthesis to ensure that cytoplasmic volume does not overcome plasma membrane capacity, and 3) the regulatory circuits responsible for partitioning cellular resources to maximize viability in response to nutrient limitation. The increasing prevalence of antibiotic resistant pathogens provides an additional and powerful motivation to understand how bacteria adapt to changing environmental conditions to maximize growth and propagation. We recently determined that pretreatment with the commercial antimicrobial triclosan increases antibiotic tolerance up to 10,000-fold, through a ppGpp-dependent mechanism. Leveraging this finding, we are employing genetic strategies to identify ppGpp-dependent changes in bacterial physiology that allow cells to survive in normally lethal concentrations of antibiotics. This work will enhance our understanding of the specific physiological adaptations underlying antibiotic tolerance and facilitate the identification of antimicrobials that can circumvent them. We are aided in all these endeavors by our multidisciplinary approach that employs a diverse array of techniques and divergent model systems, as well as an extensive network of close colleagues and collaborators with whom we enjoy a free exchange of reagents and ideas. These advantages allow us to answer previously intractable questions in physiology and homeostatic control that are relevant to organisms throughout the tree of life.